w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres Health risk assessment of organic micropollutants in greywater for potable reuse Ramiro Etchepare a,b,*, Jan Peter van der Hoek c,d Laborat orio de Tecnologia Mineral e Ambiental, Departamento de Engenharia de Minas, PPGE3M, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, Brazil b CAPES Foundation, Ministry of Education of Brazil, Brası́lia DF 70.040-020, Brazil c Delft University of Technology, Department Water Management, Stevinweg 1, 2628 CN Delft, The Netherlands d Waternet, Strategic Centre, Korte Ouderkerkerdijk 7, 1096 AC Amsterdam, The Netherlands a article info abstract Article history: In light of the increasing interest in development of sustainable potable reuse systems, Received 31 May 2014 additional research is needed to elucidate the risks of producing drinking water from new Received in revised form raw water sources. This article investigates the presence and potential health risks of 11 August 2014 organic micropollutants in greywater, a potential new source for potable water production Accepted 21 October 2014 introduced in this work. An extensive literature survey reveals that almost 280 organic Available online 12 November 2014 micropollutants have been detected in greywater. A three-tiered approach is applied for the preliminary health risk assessment of these chemicals. Benchmark values are derived Keywords: from established drinking water standards for compounds grouped in Tier 1, from litera- Greywater ture toxicological data for compounds in Tier 2, and from a Threshold of Toxicological Organic micropollutants Concern approach for compounds in Tier 3. A risk quotient is estimated by comparing the Risk assessment maximum concentration levels reported in greywater to the benchmark values. The results Potable reuse show that for the majority of compounds, risk quotient values were below 0.2, which Toxicological data suggests they would not pose appreciable concern to human health over a lifetime exposure to potable water. Fourteen compounds were identified with risk quotients above 0.2 which may warrant further investigation if greywater is used as a source for potable reuse. The present findings are helpful in prioritizing upcoming greywater quality monitoring and defining the goals of multiple barriers treatment in future water reclamation plants for potable water production. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Treatment of wastewater for potable reuse is an emerging strategy being implemented worldwide to supplement water resource portfolios, especially in arid and semi-arid regions, coastal communities faced with saltwater intrusions and regions where the quantity and/or quality of the water supply may be compromised. Many examples of potable reuse treatment trains are reported throughout the world and rio de Tecnologia Mineral e Ambiental, Departamento de Engenharia de Minas, PPGE3M, Universidade * Corresponding author. Laborato Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, Brazil. E-mail addresses: [email protected] (R. Etchepare), [email protected], [email protected] (J.P. van der Hoek). http://dx.doi.org/10.1016/j.watres.2014.10.048 0043-1354/© 2014 Elsevier Ltd. All rights reserved. w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 recent discussions among water reuse experts have addressed the reliance on the existing systems to produce acceptable and safe water to consume (Rodriguez et al., 2009; Tchobanoglous et al., 2011; Pisani and Menge, 2013; Gerrity et al., 2013). Due to an expected higher level of initial contamination in the source wastewater in comparison to conventional source waters, potable reuse systems are being scrutinized more carefully by water regulators. Accordingly, multi-barrier treatment systems are being applied to attain high levels of chemical and microbial contaminant removal and to satisfy established drinking water regulations. The evaluation of potable reuse schemes should be in line with the World Health Organization guidelines for Water Safety Plans (WSP), which are usually applied for conventional drinking water supplies (WHO, 2011). WSP are based on the human health risk assessment of the potable water supply chain and take into consideration the hazards within the system, from the catchment to the consumer, in relation to the risk of producing unsafe water. Although in most cases pathogen removal requirements drive unit process selection and integration, another important major public health concern is the potential health impacts from long-term, and in some cases, shortterm exposure to low concentration of chemicals and micropollutants present in the reclaimed water (WHO, 2011). Therefore it is important to characterize contaminant loads and associated risks for all potential drinking water sources, to adequately determine total removal required, identify appropriate treatment trains and ultimately satisfy public health criteria. Municipal wastewater treatment plant (WWTP) effluents have been the main source of water for potable reuse schemes in large-scale installations (Gerrity et al., 2013). However, a general trend is visible towards more decentralized and closed loop (onsite) systems as separating wastewater at the source and treating separately the different flows will offer possibilities to recover clean water, nutrients and energy (Jefferson et al., 2000; Cook et al., 2009; van der Hoek et al., 2014). An example of this is in the urban (domestic) environment, where “green buildings” are being commissioned in growing number (Zuo and Zhao, 2014) and water efficiency is accomplished through the collection, treatment and reuse of rainwater, black water and greywater (Johnson, 2000). Additionally, individual or cluster of housing estates and isolated communities, where there is no connection to the public water supply and sewerage, may be benefitted with more readily available sources of water for potable uses (Mwenge Kahinda et al., 2007; Cook et al., 2009). In the present paper, greywater (GW), used here to refer to domestic wastewater excluding any input from toilets (Jefferson et al., 2000), is introduced as an alternative potential source of water for potable reuse. GW has been estimated to account for about 60e80% of domestic wastewater ndez Leal, 2010), yet, its chem(Eriksson et al., 2002b; Herna ical nature is quite different. For example, the COD:BOD ratio can be as high as 4:1 (Boyjoo et al., 2013), indicating a high chemical content. It must also be pointed out that GW can be highly variable in composition, being highly dependent on the activities in the household, as well as the inhabitants' lifestyles and use of chemical products. Many previous 187 works have been published on the characteristics of GW in relation to conventional physical (temperature, colour, turbidity, electrical conductivity, suspended solids), chemical (BOD, COD, TOC, pH, nutrients, heavy metals) and microbiological (bacteria, protozoa, viruses, helminths) parameters and were recently reviewed and compiled by Boyjoo et al. (2013). Despite its much lower pathogen content (absence of feces) and organic matter content, surprisingly, GW has only been proposed for non-potable reuse applications, especially irrigation (Surendran and Wheatley, 1998; Smith and BaniMelhem, 2012; USEPA, 2012; Alfiya et al., 2013). Therefore the associated risks are generally divided into two categories: environmental risks and human health risks. Environmental risk assessments (ERA) related to detrimental effects of reclaimed water on soil characteristics (Travis et al., 2010; Turner et al., 2013), plants growth (phytotoxicity e Garland et al., 2000; Pinto et al., 2010), surface/groundwater quality and aquatic/terrestrial organisms (van Wezel and Jager, 2002; Eriksson et al., 2006) are highly important to address environmental contamination issues. Eriksson et al. (2002b) is one of the scarce studies addressing ERA of organic micropollutants (OMPs) present in GW. Since using reclaimed GW for toilet flushing and car washing is also becoming common, more information is available regarding (microbial) health risks for non-potable reuse (Dixon et al., 1999; Maimon et al., 2010; O'Toole et al., 2012; Barker et al., 2013). Nevertheless, the main challenge still waiting for advanced research development is to turn GW into potable water quality (Oron et al., 2014) and very few studies have investigated the nature, loads and associated health risks of OMPs in GW related to the use of GW as a new source for drinking water production. The latter consists the focus of the present study. At Delft University of Technology, in the Netherlands, a team of scientists, students and companies is working on the Green Village, a temporary pilot site on the campus, which will be used to test new technologies prior to their implementation in the development of the Green Campus, a more ambitious project planned at the University (van der Hoek et al., 2014). The Green Village will not be connected to water supply, the sewerage and cable systems. The aim is to develop it as an autarkic and decentralized system, producing its own potable water (from GW) and electricity, and clean its organic waste streams in a sustainable way. The present work is a first attempt, undertaken as part of the Green Village project, at compiling a hazard assessment and risk characterization to identify and understand the risks of potable water production from GW due to the presence of OMPs. Although most studies investigating GW reuse and associated risks have focused on non-potable applications and conventional water quality parameters, this work is intended to provide in-depth and up-to-date compiled data on OMPs found in GW. This paper includes a preliminary health risk assessment (screening level) by means of a theoretical and empirical framework (three-tiered approach) of OMPs that may pose a risk to human health in reclaimed potable water and ends with a discussion of the suitability of treatment barriers to mitigate problematic compounds. In part the present study is aimed at helping prioritize further investigations in this subject. 188 2. w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 Materials and methods If GW is to be treated and reused as potable water, a preliminary health risk assessment has to be conducted to identify and determine which OMPs, at the concentrations present in GW, may pose a potential health risk if not properly removed. The present work includes a risk characterization conducted in four consecutive steps. First, an extensive literature review on the presence and concentrations of OMPs in GW was conducted. Second, solute properties of the identified compounds were obtained in order to prioritize the most relevant and problematic compounds and exclude from the analysis those that are expected to be easily removed in conventional water and wastewater treatment plants. Third, a three-tiered approach was applied to derive benchmark values for the compounds with the aid of either statutory drinking water guidelines or toxicological threshold values. Finally, measured maximum GW concentrations reported were compared to the respective benchmark values and a risk quotient (RQ) was calculated. The detailed methodology used for each of these steps is described in Sections 2.1e2.4. and illustrated in Fig. 1. Mixture interactions were not quantified since the risk assessment methods for compounds with different mode of action are a complex matter still under debate. 2.1. Presence of organic micropollutants in greywater A comprehensive literature review on the presence and concentrations of OMPs in GW was performed. The survey did not include organic macro-pollutants, inorganic compounds such as nutrients and metals since they have been extensively studied elsewhere, but was confined to organic chemicals present in micro and nano-scale concentrations. The review covered the period from 1991 to 2014, by consulting published (inter)national articles, conference proceedings, academic theses and official reports. 2.2. Selection of compounds for assessment As it is not feasible to include every chemical in a toxicological assessment, the OMPs identified in GW were prioritized based on their ability to easily pass conventional water treatment barriers, as components not removed in conventional systems are likely to pose the most threat in potable reuse of GW. The n-octanol-water partition coefficient (log Kow) is a solute property related to hydrophobicity which has been used as log cut-off to prioritize compounds in toxicological assessments (Schriks et al., 2010). Compounds with a log Kow above 3 are less likely to pass water treatment plants that include an activated-carbon adsorption stage than those with lower values (Westerhoff et al., 2005). pH-corrected log Kow values are referred to as log D or distribution coefficient. The log D appears to be a more accurate and conservative tool to predict the adsorption of ionic solutes than the log Kow (Hu et al., 1997; Ridder et al., 2010). For neutral solutes, log Kow ¼ log D, but for ionic solutes log D < log Kow. In the present work, log D values were obtained with the aid of the estimation program Marvin Sketch 6.2 and compounds with a log D 3 were excluded from further assessment. An exception was made for 4 alkylphenol ethoxylates (octylphenol tetraethoxylate; octylphenol hexa-ethoxylate; octylphenol heptaethoxylate; and octylphenol octa-ethoxylate) which were not found on the estimation software. For these compounds the log D values were obtained from literature (Ahel and Giger, 1993). 2.3. Derivation of benchmark values with a three-tiered approach Due to the potential toxicity of low doses of OMPs after mid-to long-term exposure and the associated threat to public health, it was necessary to determine the concentrations of the selected contaminants at which potential adverse health effects may occur. A three-tiered approach, as similarly Fig. 1 e Flow chart indicating the risk assessment conducted in the present study. GW, greywater; Log D, distribution coefficient; RQ, risk quotient. 189 w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 proposed by Rodriguez et al. (2007), was applied in order to derive benchmark values. Compounds with an established drinking water guideline or standard value, were allocated to “Tier 1”. Compounds without drinking water standards, but for which toxicity information is available were allocated to “Tier 2”. Those compounds for which toxicity information is not available were allocated to “Tier 3”. 2.3.1. Tier 1: regulated compounds Conventionally, raw and treated potable water quality have been analysed by comparing the measured concentration of a particular substance or parameter with the respective benchmark value based on drinking water standards or guidelines. Because different states and nations regulate different contaminants or may assign their own standard values for the same contaminant, it is important to define the guidelines pertinent to a specific context. For the risk assessment of potable reuse of GW in the Netherlands, the applicable maximum contaminant levels (benchmark values) were extracted from the following drinking water guidelines, in order of priority: the Dutch Drinking Water Decree (Staatsblad, 2011), the Guidelines for Drinking Water Quality (WHO, 2011), the European Council Directive 98/83/EC (EC, 1998) and the 2011 Edition of the Drinking Water Standards and Health Advisories (USEPA, 2011). However, since the established standards for the parameters “pesticides” and “other anthropogenic compounds” in the Dutch Drinking Water Decree were considered too generic to be used in the present risk assessment, their respective target values were not used to derive benchmark values for pesticides and anthropogenic compounds. These compounds were assessed individually. available, a provisional TDI was derived based on the lowest (sub) chronic no observed (adverse) effect levels (NO(A)ELs) obtained in rodent studies divided by an assessment factor (AF) of either: 100 e includes combined factor of 10 for interspecies extrapolation and factor of 10 for inter-individual differences, 200 e includes an additional factor of 2 to extrapolate from subchronic to chronic exposure, or 600 e includes an additional factor of 6 to extrapolate from subacute to chronic exposure, depending on which was most applicable to the data available (Van Leeuwen and Vermeire, 2007). Toxicological threshold values refer to the daily exposure likely to be without deleterious effects in humans and therefore cannot be taken directly as drinking water standards but instead must be used to derive benchmark values as described by the WHO (2011). In the present study the benchmark values for drinking water were calculated using Equation (1). This method allocates 20% of the reference intake value (TDI/ADI/ RfD) for drinking water, to allow for exposure from other sources, then multiplies this allocation by the typical average body weight of an adult (60 kg) and divides it by a daily drinking water consumption of 2 L. Equation (2) was used to calculate the benchmark value corresponding to a conservative cancer risk of 105 for carcinogenic compounds which have not been assigned a toxicological threshold value but have a reported oral slope factor (SF) value instead (WHO, 2011). Benchmark value ¼ 2.3.2. Tier 2: unregulated compounds with toxicity value The first step of Tier 2 was to obtain toxicological threshold values for the assessed compounds expressed as TDI (tolerable daily intake), ADI (acceptable daily intake) and/or RfD (reference dose) from data sets and documents available from World Health Organization (WHO), U.S. EPA and other reliable (inter)national sources which are presented in Table 1. If not T x bw x P C (1) Where: T ¼ toxicological threshold value (TDI/ADI/RfD) bw ¼ body weight (60 kg) P ¼ fraction of the TDI allocated to drinking water (20%) C ¼ daily drinking water consumption (2 L) Table 1 e Sources to obtain toxicological threshold values. Sources of toxicological assessment data Environmental Health Criteria monographs (WHO) European Comission e Health and Consumer Protection (ECHCP) European Comission e Scientific Committee on Health and Environmental Risks (SCHER) European Medicines Agency (EMA) European Safe Food Authority (EFSA) Joint FAO/WHO Expert Committee on Food Additives (JECFA) Organization for Economic Cooperation and Developmente Exisiting chemicals database (OECD) The German Federal Institute for Risk Asessment (BFR) The Scientific committee on occupational exposure limits (SCOEL) U.S. EPA Integrated Risk Information System (EPA-IRIS) URL http://inchem.org/pages/ehc.html http://ec.europa.eu/dgs/health_consumer/dyna/press_room/index_ en.cfm http://ec.europa.eu/health/scientific_committees/environmental_ risks/index_en.htm http://www.ema.europa.eu/ema/ http://www.efsa.europa.eu/ http://inchem.org/pages/jecfa.html http://webnet.oecd.org/hpv/ui/Search.aspx http://www.bfr.bund.de/de/start.html http://ec.europa.eu/social/main.jsp? catId¼148&langId¼en&intPageId¼684 http://cfpub.epa.gov/ncea/iris/index.cfm?fuseaction¼iris. showSubstanceList&list_type¼alpha&view¼A 190 w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 Benchmark value ¼ bw Risk level C SF (2) presumed to present less appreciable concern to human health. Where: Risk level ¼ 105 SF ¼ Slope factor 2.3.3. Tier 3: compounds without toxicity value For compounds without toxicity information, target values were derived from a Threshold of Toxicological Concern (TTC) approach. The TTC is a conservative level of human intake or exposure that is considered to be of negligible risk to human health, despite the absence of chemical-specific toxicity data. The widely accepted TTC values proposed by Munro et al. (1996) and Kroes et al. (2004) are set as: 0.0025 mg/kg bw/day for substances that raise concern for potential genotoxicity; 0.3 mg/kg bw/day for organophosphates; 1.5, 9 and 30 mg/kg bw/day for Cramer class III, II and I substances, respectively. Thus, these values were applied for the present Tier 3 compounds. The thresholds for non-genotoxic compounds were elaborated using a dataset published by Munro et al. (1996), related to chemical classes as defined by Cramer et al. (1978) and are based on the 5th percentiles of NOELs covering chronic oral exposure. Possible genotoxic compounds and the Cramer class classification of compounds were identified in the present work through structural alerts aided by the OECD QSAR 3.2 application toolbox (URL 1). The present approach also considered the exclusion of compounds for which no TTC could be derived such as high potency carcinogens (i.e. aflatoxin-like, azoxy- or N-nitrosocompounds, benzidines, hydrazines), metal containing compounds, proteins, steroids, polyhalogenated-dibenzodioxin, -dibenzofuran, and ebisphenyl (Kroes et al., 2004). The TTC values were further translated to benchmark values by taking into account the body weight and daily ingestion of drinking water (Equation (3)). The same body weight (60 kg), allocation factor (20%) and water consumption rate (2 L) of Tier 2 were applied in Equation (3). Benchmark value ¼ 2.4. ðTTC valueÞ bw P C (3) Calculation of a risk quotient To evaluate the potential health risks and toxicological relevance of the assessed compounds, the maximum concentration levels identified in GW were divided by the benchmark value and expressed as a RQ. Compounds with a RQ 1 may be of potential human health concern if treated GW were to be consumed over a lifetime period. These compounds would be of high-priority at the selection and design of future GW treatment plants for potable water production. As similarly proposed by Schriks et al. (2010), compounds in GW with a RQ value 0.2 and <1, are considered to also warrant further investigation. Compounds in GW with a RQ value <0.2 are 3. Results 3.1. Organic micropollutants in greywater OMPs became a focus for GW research in the 1990's after two articles (Burrows et al., 1991; Santala et al., 1998) reported the presence of detergents and long-chain fatty acids detected through a GCeMS screening. A more comprehensive study in this field of research, which identified as many as 900 xenobiotic organic compounds (XOCs) as potentially present in GW, was performed by Eriksson et al. (2002a), using tables of contents of Danish household products (bathroom and laundry chemicals). The XOCs are expected to be present in GW because they originate from the various chemicals and personal care products used in households such as cleaning agents (detergents, soaps, shampoos), fragrances, UV-filters, perfumes and preservatives. Subsequent screening of bathroom GW from an apartment building in Denmark confirmed almost 200 different XOCs (Eriksson et al., 2003). However, as the study also detected some unexpected chemicals not directly connected to household chemicals (e.g. flame retardants and illicit drugs), it can be concluded that an inventory of the use of household chemicals cannot compensate for a full characterization of the compounds actually present in GW. In a later study investigating the concentrations of several selected organic hazardous substances in GW from housing areas in Sweden, Palmquist & Hanæus (2005, 2006) found that 46 out of more than 80 organic substances were present in concentrations above the detection limits. Quite recently, Donner et al. (2010) reviewed the knowledge with respect to the presence of XOCs in GW and investigated the sources, presence and potential fate of xenobiotic micropollutants in on-site GW treatment systems. However, Donner's investigation focused on non-potable reuse of GW and was limited to a few compounds selected from those listed either as Priority Substances or Priority Hazardous Substances under the European Water Framework Directive (WFD) (EU, 2000). So far the WFD has established environmental quality standards (EQS) for 41 dangerous chemical substances (33 of them classified as priority substances). However, these are only a fraction of the compounds that are potentially hazardous as this list does not include, for instance, any pharmaceutical compounds or personal care products. In spite of these findings, the number of publications on the monitoring and analysis of OMPs in GW is still scarce. There are, to the best of our knowledge, 12 published studies on this topic, where GW was produced, sampled and analysed from 7 different locations (5 housing estates, 1 camping site and 1 sport club) spread in Sweden, Denmark and the Netherlands (Eriksson et al., 2003; Andersson and Dalsgaard, 2004; Nielsen and Pettersen, 2005; Palmquist and Hanæus, 2005, 2006; Larsen, 2006; Ledin et al., 2006; Andersen et al., ndez Leal et al., 2010; Eriksson et al., 2009; Revitt 2007; Herna et al., 2011; Temmink et al., 2011). In total, 278 OMPs have been detected in GW considering all available literature data. 191 w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 Table 2 e Selected OMPs, maximum detected levels and calculated RQ values. Compounds Tier 1 Benzene Dichloromethane Ethylbenzene Pentachlorophenol Trichloromethane Tier 2 1,3-Dioxolane 1-Dodecanamine, N,N-dimethyl2,4,6-Trichlorophenol 2,4-Dichlorophenol 2-Ethyl-1-hexanol 2-Hexanone 2-Methylphenol 2-Phenyl-5-benzimidazolesulfonic acid 3,4-Dimethylphenol 3-Methylphenol 4-Methyl-phenol Maximum detected level, mg L1 9.85 4.4 2.1 0.04 250 Drinking water standard/toxicity threshold value Source Benchmark value, mg L1 1 mg L1 5 mg L1 300 mg L1 1 mg L1 300 mg L1 Staatsblad (2011) USEPA (2011) WHO (2011) USEPA (2011) WHO (2011) 1 5 300 1 300 9.85 0.88000 0.00700 0.04000 0.83333 75 mg/kg bw/day 50 mg/kg bw/day 0.011 per mg/kg bw/day 0.003 mg/kg bw/day 0.5 mg/kg bw/day 0.005 mg/kg 0.05 mg/kg bw/day 40 mg/kg bw/day 750 500 25 18 6 30 300 30,000 0.00227 0.01480 0.00400 0.00889 1.41667 0.02000 0.00080 0.00051 6 300 1500 0.00833 0.01967 0.11333 300 12,000 600 30,000 10,000 1000 0.00500 0.00004 0.03450 0.00002 0.00010 0.01700 12,000 6000 72,000 3000 600 6 4800 300 1500 10,000 15,000 450 3000 120 24,000 10,000 120 4.8 600 12,000 480 78 0.00095 0.00008 0.00021 0.00093 0.00167 0.50 0.00792 0.02967 0.00953 0.00410 0.00007 0.00133 0.00513 0.01583 0.00136 0.00370 0.00035 0.25000 0.03500 0.00175 0.00292 0.00513 9 9 9 180 180 180 180 180 180 180 180 180 0.13333 0.06667 0.01778 0.00056 0.00222 0.00167 0.01000 0.13778 0.00389 0.00167 0.00833 0.00778 0.05 5.9 170 0.001 mg/kg bw/day 0.05 mg/kg bw/day 50 mg/kg bw/day Acetaminophen Anise camphor Benzalkonium chloride Benzoic acid Benzoic acid, 4-hydroxyButylparaben 1.5 0.5 20.7 0.5 1 17 0.05 mg/kg bw/day 2 mg/kg bw/day 0.1 mg/kg bw/day 5 mg/kg bw/day 1000 mg/kg bw/day 100 mg/kg bw/day Camphor Carvone Citric acid Citronellol Coumarin Dibutyl tin Diethyl phthalate Dihydromyrcenol Dodecanamide, N,N-bis(2-hydroxyethyl)Ethylparaben Eugenol Isoeugenol Linalool Malathion Menthol Methylparaben Naphthalene Nicotine Phenol Propylparaben Toluene Tri(2-chloroethyl) phosphate Tier 3 1,2-Ethanediamine, N-ethyl1,8-Nonanediol, 8-methyl2,5-Dichlorophenol 2,5-Dimethylphenol 2,6-Dimethylphenol 2-Hexanol 2-Methyl-butanoic acid, methyl ester 2-Phenoxy ethanol 3-Hexanol 3-Hexanone 3-Methyl-butanoic acid, methyl ester 4-Heptanone 11.4 0.5 15 2.8 1 3 38 8.9 14.3 41 1 0.6 15.4 1.9 32.6 37 0.042 1.2 21 21 1.4 0.4 2 mg/kg bw/day 1 mg/kg bw/day 1200 mg/kg bw/day 0.5 mg/kg bw/day 0.1 mg/kg bw/day 1 mg/kg bw/day 0.8 mg/kg bw/day 10 mg/kg bw/day 50 mg/kg bw/day 10 mg/kg bw/day1 2.5 mg/kg bw/day 0.075 mg/kg bw/day 0.5 mg/kg bw/day 0.02 mg/kg bw/day 4 mg/kg bw/day 10 mg/kg bw/day1 0.02 mg/kg bw/day 0.0008 mg/kg bw/day 0.1 mg/kg bw/day 2 mg/kg bw/day 0.08 mg/kg bw/day 13 mg/kg bw/day EFSA (NOAEL); AF ¼ 600 OECD (NOEL); AF ¼ 600 EPA-IRIS (SF) EPA-IRIS (RfD) JECFA (ADI) EPA-IRIS (RfD) EPA-IRIS (RfD) ECHCP (NOAEL); AF ¼ 200 EPA-IRIS (RfD) EPA-IRIS (RfD) EPA report (NOAEL); AF ¼ 200 EMA (ADI) JECFA (ADI) BFR (ADI) JECFA (ADI) OECD (NOAEL); AF ¼ 600 Daston et al. (2004) (NOEL); AF ¼ 600 EFSA (TDI) JECFA (ADI) OECD (NOAEL); AF ¼ 100 JECFA (ADI) EFSA (TDI) WHOeIPCS (2006) (TDI) EPA-IRIS (RfD) JECFA (NOAEL); AF:200 EFSA (NOAEL); AF ¼ 200 EFSA (NOAEL); AF ¼ 600 JECFA (ADI) EMA (ADI) JECFA (ADI) EPA-IRIS (RfD) JECFA (ADI) EFSA (NOAEL); AF ¼ 600 EPA-IRIS (RfD) EFSA (ADI) WHO (ADI) JECFA (ADI) EPA-IRIS (RfD) SCHER (TDI) 1.2 0.6 0.16 0.1 0.4 0.3 1.8 24.8 0.7 0.3 1.5 1.4 1.5 mg/kg bw/day 1.5 mg/kg bw/day 1.5 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day TTC (Cramer TTC (Cramer TTC (Cramer TTC (Cramer TTC (Cramer TTC (Cramer TTC (Cramer TTC (Cramer TTC (Cramer TTC (Cramer TTC (Cramer TTC (Cramer 1.7 7.4 0.10 0.16 8.5 0.6 0.24 15.3 RQ class class class class class class class class class class class class III) III) III) I) I) I) I) I) I) I) I) I) (continued on next page) 192 w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 Table 2 e (continued ) Compounds 4-Methoxy-benzoic acid 4-Methyl-pentanoic acid, methyl ester 6-Methyl-5-hepten-2-one Acetamide Acetic acid, phenoxyBenzenesulfonic acid, methyl ester Butanoic acid, butyl ester Caffeine Decanamide, N-(2-hydroxyethyl)Decanoic acid Dimethyl phthalate Dodecanoic acid Eucalyptol Geraniol Hexanoic acid, methyl ester Homomyrtenol Hydroxycitronellol Indole Isobutylparaben Methyl dihydrojasmonate Mono 2-ethylhexyl phthalate Monobutyl tin Monooctyl tin Octanoic acid Pentanoic acid, methyl ester Phenylethyl alcohol Propanoic acid, 2-methyl-, 2,2-dimethyl-1-(2-hydroxy1-methylethyl)propyl ester Propanoic acid, 2-methyl-, 3-hydroxy-2,2,4-trimethylpentyl ester Salicylic acid Sulphuric acid, dimethyl ester Terpineol Tetracanoic acid a-Methyl-benzene methanol Maximum detected level, mg L1 12.7 1.1 0.1 8.6 4 1.1 0.9 0.5 3.2 1.2 4.9 680 0.1 0.8 10.1 0.9 0.2 3.8 8 3.9 1.7 0.99 0.1 3 1.1 0.6 1.1 0.3 0.6 0.1 1.2 2808 0.1 Drinking water standard/toxicity threshold value 30 mg/kg bw/day 1.5 mg/kg bw/day 30 mg/kg bw/day 1.5 mg/kg bw/day 30 mg/kg bw/day 0.0025 mg/kg bw/day 30 mg/kg bw/day 1.5 mg/kg bw/day 1.5 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 1.5 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 1.5 mg/kg bw/day 30 mg/kg bw/day 1.5 mg/kg bw/day 30 mg/kg bw/day 1.5 mg/kg bw/day 1.5 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 1.5 mg/kg bw/day TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC TTC 1.5 mg/kg bw/day 30 mg/kg bw/day 0.0025 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day 30 mg/kg bw/day The full list of the OMPs identified and their concentrations is provided in supplementary information, Table S1. Identified compounds were grouped into eleven substance classes: 1) Plasticisers and softeners; 2) Preservatives; 3) UV filters; 4) Surfactants and emulsifiers; 5) Flavours and fragrances; 6) Polycyclic aromatic hydrocarbons (PAHs); 7) Polychlorinated biphenyls (PCBs); 8) Solvents; 9) Brominated flame retardants; 10) Organotin compounds; and 11) Miscellaneous. 3.2. Source Selection of compounds The outcome of the prioritization of OMPs found in GW resulted in the identification of 89 compounds (log D < 3) out of the original list. These compounds were selected for further assessment. Of these 89 chemicals surfactants contributed 5, fragrances and flavours 26, plasticisers 4, preservatives 17, solvents 10, organotin compounds 3, UV filter 1, PAH 1, and other miscellaneous compounds 22. These OMPs and their respective CAS numbers and log D values are listed in Table S2 (supplementary data). (Cramer class I) (Cramer class III) (Cramer class I) (Cramer class III) (Cramer class I) (potential genotoxic) (Cramer class I) (Cramer class III) (Cramer class III) (Cramer class I) (Cramer class I) (Cramer class I) (Cramer class III) (Cramer class I) (Cramer class I) (Cramer class I) (Cramer class I) (Cramer class III) (Cramer class I) (Cramer class III) (Cramer class I) (Cramer class III) (Cramer class III) (Cramer class I) (Cramer class I) (Cramer class I) (Cramer class III) Benchmark value, mg L1 RQ 180 9 180 9 180 0.15 180 9 9 180 180 180 9 180 180 180 180 9 180 9 180 9 9 180 180 180 9 0.07056 0.12222 0.00056 0.95556 0.02222 7.33333 0.00500 0.05556 0.35556 0.00667 0.02722 3.77778 0.01111 0.00444 0.05611 0.00500 0.00111 0.42222 0.04444 0.43333 0.00944 0.11 0.01111 0.01667 0.00611 0.00333 0.12222 TTC (Cramer class III) 9 0.03333 TTC TTC TTC TTC TTC 180 0.15 180 180 180 (Cramer class I) (potential genotoxic) (Cramer class I) (Cramer class I) (Cramer class I) 0.00333 0.66667 0.00667 15.6 0.00056 3.3. Preliminary health risk assessment of selected OMPs in GW The final list of OMPs in GW with their respective benchmark values and RQ values is provided in Table 2. For only 5 compounds (benzene, dichloromethane, ethylbenzene, pentachlorophenol and trichloromethane) statutory drinking water guideline values were available and these compounds were grouped into Tier 1. The benchmark values of Tier 1 ranged from 1 mg/L (benzene) to 300 mg L1 (ethylbenzene and trichloromethane, respectively) and originated from the Dutch Drinking Water Decree, the WHO Guidelines for Drinking Water Quality and the USEPA, according to the order of priority set in the present work. Toxicological data were found for 39 compounds (Tier 2). An established TDI, ADI or RfD was available for 27 compounds and in 11 cases when there was no TDI, ADI or RfD available, an established NO(A)EL was utilized to derive a TDI value with the aid of assessment factors. Specifically for the carcinogenic 2,4,6-trichlorophenol there was an SF available from EPA-IRIS. The remaining 45 compounds with no toxicological data were grouped into Tier 3. w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 The latter comprised 29 compounds allocated to Cramer class I, 14 compounds allocated to Cramer Class III and 2 compounds with genotoxic structural alerts. Calculated benchmark values varied from 0.15 mg L1 (for the possible genotoxic benzenesulfonic acid, methyl ester and sulphuric acid, dimethyl ester) to 72,000 mg L1 (for the preservative citric acid). The highest observed benchmark values (eight of them >10,000 mg L1) referred to preservatives and fragrances/flavours, which in general are also chemicals utilized as food additives. The lowest observed benchmark values related to compounds allocated to Tier 3 (from 0.15 to 180 mg L1), with exception for benzene (1 mg L1), dichloromethane (5 mg L1) and pentachlorophenol (1 mg L1) in Tier 1; 2,4,6-trichlorophenol (25 mg L1), 2,4-dichlorophenol (18 mg L1), 2-ethyl-1-hexanol (6 mg L1), 2-hexanone, 3,4dimethylphenol (6 mg L1), dibutyl tin (6 mg L1), nicotine (4.8 mg L1), and tri(2-chloroethyl) phosphate (78 mg L1) in Tier 2. For 5 compounds the RQ value was above 1, namely: benzene (Tier 1); 2-ethyl-1-hexanol (Tier 2); benzenesulfonic acid methyl ester; dodecanoic acid; and tetracanoic acid (Tier 3). Accordingly, these compounds may be of potential human health concern if not reduced in treatment barriers and are considered to be of higher priority for further studies on the risk assessment and the selection of technologies to be applied in future GW treatment plants for drinking water production. For 9 compounds (dibutyl tin; dichloromethane; trichloromethane; nicotine; acetamide; indole; decanamide, N-(2-hydroxyethyl)-; sulphuric acid, dimethyl ester; and methyl dihydrojasmonate), the RQ value was above 0.2 (and below 1). These compounds are also considered to warrant further investigation. 4. Discussion Potable reuse of GW is a novel and potentially beneficial research topic given the increasingly urgent need to identify and validate new raw water sources for safe drinking water production worldwide. An important concern in the development of GW potable reuse schemes appears to be the lack of knowledge about the presence and risks of OMPs. The occurrence of OMPs has been much better characterized in WWTP influents and effluents and in surface waters than in GW (Pal et al., 2010; Deblonde et al., 2011; Luo et al., 2014), and very little is known about OMPs in industrial wastewaters. WWTPs that treat domestic (household) sewage, hospital effluents, industrial wastewaters, as well as wastewaters from livestock and agriculture are considered to be the main source of OMPs to aquatic systems (Kasprzyk-Hordern et al., 2009). Most of previous studies on GW characterization and treatment have been limited to the assessment of conventional water quality parameters for non-potable reuse applications. Accordingly, the first challenge facing those who wish to treat GW to potable water quality is to identify the chemicals which potentially represent a threat to human health in future applications. The present study combined available data in literature with risk characterization methods in order to improve our understanding regarding the presence of OMPs in GW and the risks they may pose to human health. 193 The results presented in Table S1 (supplementary data) confirmed the presence of OMPs directly associated with household chemicals, especially personal care products. Several miscellaneous compounds, probably indirectly associated with household chemicals have also been identified (e.g. brominated flame retardants, organotin compounds, and drugs). Nevertheless, pharmaceuticals active compounds, which have been consistently detected in hospital effluents (Verlicchi et al., 2010) and WWTPs (Deblonde et al., 2011; Luo et al., 2014) and raised environmental and human health concern due to their persistency and potential in endocrine disruption (Daughton and Ternes, 1999), were virtually not present. Two exceptions were the pharmaceuticals acetaminophen and salicylic acid, but maximum detected levels in GW (1.5 mg L1 and 0.6 mg L1, respectively) are about 500 (acetaminophen) and 3500 (salicylic acid) times lower than the corresponding maximum levels reported in WWTP effluents (Pal et al., 2010 e Table 3). As administrated pharmaceutical compounds are excreted from the human body via feces and urine, separate collection and treatment of GW in households can contribute to keeping these substances away from reclaimed (potable) water. Table 3 compares the concentrations of some of the OMPs compiled in the present study with maximum concentrations reported for WWTP influents and effluents (based on recent review papers/compiled literature data). Besides pharmaceuticals, in general, much higher loads of OMPs associated to industrial chemicals and wastewaters are observed in WWTPs influents (among them: bisphenol-A ¼ 11.8 mg L1; 4nonylphenol ¼ 101.6 mg L1; 4-octylphenol ¼ 8.7 mg L1; dibutylphtalate ¼ 46.8 mg L1) when compared to GW (bisphenol-A ¼ 1.2 mg L1; 4-nonylphenol ¼ 38 mg L1; 4octylphenol ¼ 0.16 mg L1; dibutylphtalate ¼ 3.1 mg L1), while concentrations of personal care products are slightly higher in GW. Intermittent contributions from agricultural and/or livestock runoff and hospital discharges may also cause spikes in pharmaceuticals and steroid hormones in WWTP influents and effluents (Verlicchi et al., 2010; Sim et al., 2011) and industrial discharges may contain organic compounds and other materials that are typically absent in GW (e.g. aminopolycarboxylate complexing agents e Reemtsma and Jekel, 2006). On the other hand, another important factor is rainfall. Kasprzyk-Hordern et al. (2009) found that the concentrations of a selection of 55 OMPs in the WWTP influent were doubled when the flow was halved during dry weather conditions, suggesting that rainwater could dilute the concentrations of the compounds within the sewage. Therefore, the common practice in potable reuse schemes of cotreatment of hospital, industrial, agriculture, stormwater and domestic wastewaters at a municipal WWTP (Gerrity et al., 2013) is not a sustainable approach for reducing the risks of OMPs because it is based on dilution of different discharges and does not provide an adequate segregation of pollutants and, in particular, of different classes of OMPs. A preliminary health-based risk assessment of 89 prioritized OMP (with log D < 3) in GW was performed to determine benchmark values. The first step was a conventional evaluation of contaminants and consisted of identifying compounds with an established drinking water guideline or standard value (Tier 1). The need to develop additional tiers arose 194 w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 Table 3 e Maximum concentrations of OMPs in GW (present study) in comparison with maximum levels reported in WWTP influents and effluents. The literature data of WWTPs were compiled from recent review papers (Pal et al., 2010; Deblonde et al., 2011; Luo et al., 2014). Compound Acetaminhophen Salicylic acid Caffeine Benzophenone Galaxolide Tonalide Triclosan 4-Nonylphenol 4-Octylphenol Bisphenol-A Butylbenzyl phtalate Di-(2-ethylhexyl) phthalate Dibutyl phthalate Diethyl phtalate Di-isobutyl phthalate Dimethyl phtalate Dimethyl phthalate Class GW (present study) (mg L1) Pharmaceutical Pharmaceutical Food additive/stimulant Personal care product Personal care product Personal care product Personal care product Surfactants Surfactants Plasticizer Plasticizer Plasticizer Plasticizer Plasticizer Plasticizer Plasticizer Plasticizer because no current guidelines exist for a majority of the chemicals identified in this study. As the fulfilment of the criteria for establishment of a guideline value may take place several years after a potential contaminant is identified (WHO, 2011), an attempt was made to characterize the risks of selected compounds with no established guidelines. There were 39 chemicals in this study for which relevant toxicity information (ADI, TDI, RfD, NOA(E)L) exists (Tier 2), thus benchmark values were derived from this available information. Health authorities recommend using maximum acceptable or tolerable levels such as ADI, RfD and TDI as guidelines for contaminants that may accumulate in the body. Since its introduction in 1957 by the Council of Europe and later by the Joint Expert Committee on Food Additives-JECFA (WHO, 2002), the ADI has been proven to be a valid and practical tool in the risk assessment and are the basis for many regulatory standards (WHO, 2011). The remaining compounds were those without established drinking water criteria or toxicity data (Tier 3). The benchmark values developed in this study for compounds in Tier 3 ranged from 0.15 to 180 mg L1. The widely accepted TTC approach used to derive these benchmark values (Kroes et al., 2004; Munro et al., 1996) was considered appropriately conservative and protective to human health, since it has been applied frequently by regulatory bodies for risk assessment of substances at low dose oral exposure for which limited or no toxicity data are present (Leeman et al., 2014; EFSA, 2012; EU, 2012). However, it should be noted that more conservative TTC approaches than the one applied in the present study have also been proposed. Mons et al. (2013), for example, set TTC values for all chemicals other than genotoxic and steroid endocrine compounds at 1.5 mg/person per day (target value in drinking water equal to 0.1 mg L1), to achieve drinking water of impeccable quality in line with the so-called Q21 approach. On the other hand, the thresholds should be as accurate as feasible and not over conservative to prevent unnecessary low thresholds. In this respect it is noted that recently new 1.5 0.6 0.5 4.9 19.1 5.8 35.7 38 0.16 1.2 9 160 3.1 38 8 4.9 4.9 WWTPs Influent (mg L1) Effluent (mg L1) 56.9 63.7 209 0.9 25 1.93 23.9 101.6 8.7 11.8 37.87 122 46.8 50.7 20.48 3.32 6.49 777 2098 43.5 0.23 2.77 0.32 6.88 7.8 1.3 4.09 3.13 54 4.13 2.58 e 0.115 1.52 thresholds have been proposed above the current (accepted) thresholds used in this study (Munro et al., 2008; Tluczkiewicz et al., 2011; Leeman et al., 2014). These new possibilities for the TTC approach must be further elucidated and validated by international regulatory agencies before they can be put into practice. Five pesticides were assessed in the present study (2,4,6trichlorophenol, 2,4-dichlorophenol, 2,5-dichlorophenol, malathion and pentachlorophenol). The benchmark values derived for them in this study ranged from 1 to 120 mg L1 and were far above the established standard (0.1 mg L1) for pesticides set by the Dutch Drinking Water Decree and the European Council Directive 98/83/EC. Although the present results suggest that these statutory standards might be overly pragmatic and stringent, it is advisable that drinking water produced from GW complies with the pesticide mandatory target value of 0.1 mg L1. The calculated RQ values for the majority of OMPs were below 1, indicating that these compounds are presumed to present little appreciable danger to human health. However, a few compounds (benzene; 2-ethyl-1-hexanol; benzenesulfonic acid, methyl ester; dodecanoic acid and tetracanoic acid) had RQ values above 1, which suggests that these compounds may pose a more appreciable concern. Further investigations should focus on reducing the concentrations of these more problematic compounds from GW by the application of advanced treatment barriers in order to reach the target safe levels. Different wastewater treatments may be appropriate only for some of these OMPs due to the variability of their physico-chemical properties (e.g. hydrophobicity, molecular weight, and chemical structure e Table S3) and therefore, a multiple barriers treatment is advisable. In Windhoek, for instance, direct drinking water reclamation from wastewater has already been applied successfully for more than 40 years based on the multiple barriers concept to reduce associated risks and improve the water quality (du Pisani and Menge, 2013). The treatment train consists of the w a t e r r e s e a r c h 7 2 ( 2 0 1 5 ) 1 8 6 e1 9 8 following partial barriers for OMPs removal: pre-ozonation, enhanced coagulation þ dissolved air flotation þ rapid sand filtration, and subsequent ozone, biological activated carbon/ granular activated carbon. Based on these considerations, to remove OMPs from GW for potable reuse, a triple barrier consisting of a membrane bioreactor (MBR, coupled with an ultrafiltration membrane), ozone-based advanced oxidation process (AOP) and activated carbon adsorption (AC) appears to be promising (van der Hoek et al., 2014). MBRs are able to effectively remove a wide spectrum of OMPs that are resistant to conventional biological processes (Tadkaew et al., 2011; Trinh et al., 2012). Ozonebased AOP and AC have demonstrated to be effective for removing the prioritized compounds found in the present ndez Leal et al., 2011; Lee et al., study (Rosal et al., 2010; Herna nchez et al., 2014). The application of AC is also 2012; Jurado-Sa supported by results obtained herein, which showed that 189 out of the 278 compounds detected in GW have Log D values above 3 (high sorption), and thus are expected to be removed by this treatment stage. In the Netherlands, this treatment train will be tested and extensively studied in the aforementioned Green Village project at Delft University of Technology. The clean water supply of its test laboratory site will be provided using GW and rainwater generated on site as raw water sources by reclaiming them in a pilot scale multiple barrier treatment concept for drinking water production. Looking towards the future, the results presented in this article can help researchers, water engineers and stakeholders to prioritize further investigations about the use of GW as potable water supply. 5. Conclusions An extensive literature review showed that, in total, 278 OMPs have been detected in GW from 7 different sites located in Denmark, Sweden and the Netherlands; The study shows a practical tool to assess the health risks of relevant OMPs by deriving benchmark values for a group of (prioritized) compounds (log D < 3); The preliminary health risk assessment, performed with the aid of a three tiered approach, showed that for only a minority of selected OMPs, established drinking water standards are available. Benchmark values for nonregulated compounds were derived based on either toxicological available data or TTC approach; The RQ values obtained (based on the maximum concentration levels detected in the limited available GW sources and on calculated benchmark values) revealed that from the toxicological point of view, the majority of assessed chemicals would not pose appreciable human health concern in an exposure scenario to drinking water over a life-time period; A group of 5 compounds with RQ value > 1 as well as 9 compounds with the RQ value between 0.2 and 1 suggest that advanced multiple treatment barriers would be required in future potable water reclamation plants to reduce the concentration of these compounds to safe levels. 195 Acknowledgements The authors wish to thank CAPES (Brazilian institution), that directly sponsored these doctoral studies at Delft University of Technology (Scholarship n 8106-13-4). 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